[FFmpeg-devel] [PATCH] avcodec/mobiclip: Check quantizer before table setup

[Paul B Mahol]

On Fri, Sep 11, 2020 at 08:09:48PM +0200, Michael Niedermayer wrote:
> On Fri, Sep 11, 2020 at 11:12:33AM +0200, Paul B Mahol wrote:
> > On Fri, Sep 11, 2020 at 01:19:59AM +0200, Michael Niedermayer wrote:
> > > Fixes: index -1 out of bounds for type 'const uint8_t [6][16]'
> > > Fixes: out of array read
> > > Fixes: shift exponent -21 is negative
> > > Fixes: 25422/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5748258226569216
> > > Fixes: shift exponent 8039082 is too large for 32-bit type 'int'
> > > Fixes: 25430/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MOBICLIP_fuzzer-5698567770210304
> > > 
> > > Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> > > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > > ---
> > >  libavcodec/mobiclip.c | 6 +++++-
> > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/libavcodec/mobiclip.c b/libavcodec/mobiclip.c
> > > index d147eddbae..13f0edc89d 100644
> > > --- a/libavcodec/mobiclip.c
> > > +++ b/libavcodec/mobiclip.c
> > > @@ -1323,6 +1323,10 @@ static int mobiclip_decode(AVCodecContext *avctx, void *data,
> > >          }
> > >      } else {
> > >          MotionXY *motion = s->motion;
> > > +        int quantizer = s->quantizer + get_se_golomb(gb);
> > > +
> > > +        if (quantizer < 12 || quantizer > 161)
> > 
> > From where this numbers come?
> 
> It comes from the implementation
> qtab is int and will overflow beyond 161

ok then.

> 
> and below 12 the shift will be negative here:
> s->qtab[1][i] = quant8x8_tab[qx][i] << (qy - 2);
> 
> 
> > I think this is too aggressive.
> 
> 
> [...]
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
> Many that live deserve death. And some that die deserve life. Can you give
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> even the very wise cannot see all ends. -- Gandalf



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